1. Field of the Invention
This invention relates generally to power amplifier control. More particularly, the invention relates to a voltage clamp for improving the transient response of a collector voltage controlled power amplifier.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, portable communication devices are becoming more and more widespread. A portable communication device includes one or more power amplifiers for amplifying the power of the signal to be transmitted from the portable communication device. In a collector voltage controlled power amplifier (PA), the output power is determined by a regulated voltage applied to the collector of a bi-polar junction transistor (or drain, if implemented as a field effect transistor (FET)) of one or more stages of the power amplifier. Ideally, this voltage is referred to as VCC and should closely follow (also referred to as “track”) an input voltage envelope control signal (referred to as VENV), according to a transfer function defined as VCC=H(VENV). The voltage VENV controls the power amplifier as a function of the peak voltage of the power amplifier determined during calibration, and corresponds to power amplifier output power.
Typically, a power amplifier requires a relatively large current source. To source this large current with minimal voltage drop from the power source (e.g., a battery having voltage VBATT) to the collector of the power amplifier, a relatively large transistor (typically a field effect transistor (FET)) is used as the output device of the voltage regulator. As current flows from the battery, the battery voltage falls to a value below the desired collector voltage of the power amplifier as determined by H(VENV). Under this condition, the voltage (VDS) across the drain and source terminals of the FET at the output of the voltage regulator is too small to maintain saturation of the FET and the voltage at the collector terminal of the power amplifier (VCC) is no longer determined by the voltage regulator, but rather by the battery voltage, VBATT. Typically, the transition from this unregulated condition back to a regulated condition (when VCC is reduced sufficiently below VBATT) is abrupt, also referred to as “switching transients,” leading to undesirable spectral components, referred to as spectral regrowth. Spectral regrowth can cause interference to other portable communication devices operating in adjacent frequency bands.
Therefore, it would be desirable to control the voltage applied to the collector of a power amplifier so that there are no abrupt transitions, thereby minimizing spectral regrowth due to switching transients.